Highly selective gas separation by two isostructural boron cluster pillared MOFs

Abstract

• Remarkable C 2 H 2 /C 2 H 4 selectivity and high C 2 H 2 capacity in BSF-9 with reasonable illustration of the C 2 H 2 and C 2 H 4 adsorption difference by modeling study. • Excellent dynamic separation performance of C 2 H 2 /C 2 H 4 in BSF-9 with good recyclability. • High CO 2 /N 2 selectivity achieved both by BSF-4 and BSF-9 through near-sieving effect. • Inverse ambient adsorption capacity of C 3 H 8 and C 2 H 6 observed in BSF-9. • Distinct C 3 H 8 adsorption kinetics observed for BSF-4 and BSF-9. Two isostructural closo -dodecaborate [B 12 H 12 ] 2- pillared metal organic frameworks (MOFs), BSF-4 and BSF-9, were synthesized and compared in the performance of selective gas adsorption and separation. BSF-9 with symmetrical interpenetration and two contracted 1D channel exhibited efficient uptake of C 2 H 2 (85.1/76.3 mLg −1 , 278/298 K) with high separation selectivity over C 2 H 4 (41.4, 298 K), outperforming BSF-4 (7.3, 298 K) and many benchmark MOFs. The selectivity of C 2 H 6 /CH 4 , CO 2 /CH 4 , CO 2 /N 2 on BSF-9 (25.2, 9.9, 56.6) and BSF-4 (19.0, 8.9, 41.7) was all relatively high and comparable. The practical gas separation ability for C 2 H 2 /C 2 H 4 mixtures on BSF-9 was confirmed by mixed gas breakthrough experiments with negligible capacity loss after 4 cycles, indicating its potential for purification of ethylene from acetylene-containing industrial gas mixtures. The C 2 H 2 and C 2 H 4 bonding sites and bonding energies within the framework of BSF-9 were further compared by density functional theory (DFT) study, indicating that C 2 H 2 can be trapped tightly between two anionic [B 12 H 12 ] 2- by four synergistic dihydrogen bonds while C 2 H 4 only interacts with single [B 12 H 12 ] 2- with weaker bonding energy. BSF-4 with asymmetrical interpenetration and single extended 1D channel showed higher selectivity of C 3 H 8 /CH 4 (138.5, 298 K) than that of BSF-9 (64.0, 298 K). Inverse ambient adsorption capacity of C 3 H 8 < C 2 H 6 was observed in BSF-9 due to the small pore window and 1D channel.